Many of the known filtration systems having flat filters, which are used, for example, for wastewater cleaning, contain filter modules with a tubular or box-shaped housing which is open at the top and bottom and in which a plurality of flat filter elements are arranged parallel to one another. The gaps between the filter elements form passages through which the flow can pass. The filter elements are embodied as pockets or cassettes in which a flexible or rigid drainage element is surrounded on both sides by a support nonwoven coated with a filter membrane—and referred to below as a membrane nonwoven. Each filter element has openings which are arranged centrally or at the edges and via which said filter element is connected to a pipe system for evacuation of the permeate fluid passing through the filter membrane. The drainage element serves as a spacer and shaping support for the membrane nonwovens and for conducting away the permeate to the openings and for evacuation thereof. The flow resistance of the drainage element influences the distribution of pressure in the interior of the filter element and, in association therewith, the efficiency of the filter.
WO 03/037489A1 (whose United States equivalent is United States Patent Publication No. 2005/000881A1) describes a filter module for cleaning wastewater, with a plurality of filter membrane pockets which have at least one opening for draining the interior thereof and are arranged vertically, parallel and preferably at the same distance from one another in a rigid holder in such a manner that a liquid can flow intensively through the gaps located between adjacent filter membrane pockets.
DE 2 213 165 discloses a filter in which the filter cartridge consists of a multi-layered cloth which is wound spirally and has two opposite, spiral end surfaces. The layers forming the cloth are sealed in such a manner that the fluid entering the filter cartridge through a spiral end surface has to flow through at least one filter surface before being able to leave the filter cartridge again. Each filter layer is located between two thin spacer layers having a high pore volume. At least one fluid-impermeable layer is placed against the surface of one of the spacer layers. By means of the use of a thin spacer layer, a compact filter cartridge having a large active filter surface per unit of volume is obtained.
U.S. Pat. No. 5,304,312 describes a sealed filter unit with a first and second end cap on the end sides, wherein the end caps have connectors for the connection of lines for an untreated liquid to be filtered and for a permeate filtered out of said untreated liquid. The filter unit contains a filter element which is arranged between the end caps and consists of a spirally wound, two-layered filter composite material which comprises a filter layer and a liquid-impermeable spacer layer. At the two opposite end sides of the spirally wound filter composite material, the edges of the filter layer are each connected in a liquid tight manner to the spacer layer adjacent on the left and right. This construction ensures that the untreated liquid supplied via the first end cap has to permeate the filter layer first in order to be removed as permeate at the second end cap.
EP 1 256 372 A2 (whose United States equivalent is United States Patent Publication No. 2002/168,491A1) teaches a filter module which comprises a capillary filter mat wound spirally around a perforated pipe. An untreated fluid to be filtered is supplied through the perforated pipe and brought into contact with the spirally wound capillary filter mat. By means of a pressure differential between the inside and outside of the capillary filters, a permeate is filtered out of the untreated fluid and removed from the interior of the capillary filters. Access to the interior of the capillary filters is provided by the two end sides of the spirally wound capillary filter mat being provided with end caps made of a castable, curable material and the end caps obtained being cut to size substantially perpendicularly to the longitudinal axis of the capillary filters.
During constant operating conditions, such as trans-membrane differential pressure, solids content and flow velocity of the untreated fluid, the capacity of a filtration system, i.e. the volume of untreated fluid filtered per hour, is proportional to the active filtration surface. Due to the production-induced limitations and to the limited mechanical load-bearing capacity of industrially manufactured filter membranes, the size of the active filtration surface of conventional filter modules upward is limited. Therefore, in order to construct filtration systems having a high filtration capacity, a large number of filter modules are connected to one another in a type of parallel connection by means of a pipeline system. Pipeline systems of this type which customarily have a multiplicity of branches and pipe couplings are costly to produce, complicated to install and increase the space required by the filtration system without contributing to the filtration capacity.